PROJECT SUMMARY/ABSTRACT In the United States Staphylococcus aureus infections are highly prevalent, both in healthcare settings and within the community. These infections range in severity from recurrent skin abscesses to life-threatening necrotizing pneumonias. A clear understanding of how S. aureus is able to subvert the host immune system is necessary in order to develop effective therapeutic approaches. Important factors contributing to S. aureus virulence are the bi-component pore forming toxins (B-PFTs). B-PFT subunits assemble on the surface of host cells and form oligomeric pores that ultimately result in cellular death. Leukocidin AB, LukAB, is the most recently identified B-PFT and kills primary human neutrophils, monocytes, macrophages and dendritic cells. Among S. aureus B-PFTs, LukAB contains the most amino-acid divergence, is the only toxin known to enhance S. aureus survival upon phagocytosis by human neutrophils, is the only toxin isolated as a dimer, and is the only B-PFT that uses a host integrin as a receptor (specifically CD11b, a component of the CD11b/CD18 integrin). Allelic variants of LukAB are also found in clinical isolates, although their functional role is unknown. Additionally, the mechanistic details of how LukAB induces cell death remain largely unknown. As such, the primary goal of this proposal is to identify the cellular signals emanating from LukAB interaction with host receptor that result in immune cell death. Experiments described in this application seek to (i) determine the role of the cytoplasmic tail of CD11b, and associated tyrosine kinases, toward LukAB-mediated cell death in human monocytes and (ii) determine the contribution of LukAB-allelic variants to immune cell response and lysis. Methods in the proposed experiments involve immunological biochemistry, mammalian and bacterial genetics, and in vitro and ex vivo cytotoxicity assays. Fully addressing these questions will provide a greater understating of the mechanistic details for an important S. aureus virulence factor and provide the necessary foundation for future development of specific inhibitors to LukAB-mediated cell death.